Conventionally, regarding a sealing apparatus, such as an oil seal for example, that seals an annular gap between a shaft and a housing that are relatively rotated, those being provided with a seal lip that is in sliding contact with the shaft surface are general. Such a sealing apparatus will be described with reference to FIG. 5. FIG. 5 is a partial cross-sectional view of a sealing apparatus according to a related art.
As illustrated, a sealing apparatus 200 according to the related art is provided with a reinforcing ring 210 and a rubber-like elastic body 220 that is baked and fixed to the reinforcing ring 210. The rubber-like elastic body 220 is provided with an outer circumference seal 221 that seals the inner circumferential surface of a shaft hole disposed in a housing not illustrated, a seal lip 222 that seals by being in sliding contact with a shaft surface not illustrated, and a dust lip 223 that seals by being in sliding contact with the shaft surface in the same manner. In the drawings, X indicates a position corresponding to the shaft surface when the sealing apparatus 200 is mounted in an annular gap between the shaft and the housing.
In such a sealing apparatus 200, in order to prevent leakage of the sealing fluid, the seal lip 222 needs to be in contact with the shaft surface at a suitable contact force (force that acts on the shaft surface in a direction perpendicular to the shaft center). The larger this contact force is, the higher the mechanical loss will be.
In order to restrain this mechanical loss, the contact force must be reduced. However, when the contact force of the seal lip 222 is reduced, the sealing performance decreases. For this reason, there is known a technique of providing the sliding contact surface of the seal lip 222 with a thread 222a that exhibits a pump function of returning the leaked sealing fluid to the sealing fluid side (See, for example, the patent document 1). However, in order to let the pump function be exhibited effectively, the distance of the thread 222a in the axial direction must be ensured to some extent. In this case, the contact area of the seal lip will be wide to some extent, so that there is a limit in reducing the contact force of the seal lip.
Also, as a method for reducing the contact force of the seal lip while maintaining the sealing performance, there is a method of reducing the rigidity of the seal lip while sufficiently ensuring the interference of the seal lip. However, in this case, turn-over of the seal lip is liable to be generated. For example, as illustrated in FIG. 5, in a structure in which the seal lip extends toward the sealing fluid side, in the case of an assembly system in which the shaft is inserted from the sealing fluid side toward the anti-sealing fluid side for assembling, the seal lip is more liable to be turned over to the inside according as the rigidity of the seal lip is lower. For this reason, there is a limit in reducing the rigidity of the seal lip and, even in the case of this method, there is a limit in reducing the contact force of the seal lip.
Patent Document 1: Japanese Patent Application Laid-Open No. 52-148757